High-sensitivity capacitive sensor circuit

ABSTRACT

A high-sensitivity capacitive sensor circuit having improved sensitivity by implementing a plurality of detection units using charging and discharging, has: an oscillation unit for generating a control clock; a first charge/discharge unit connected to a sensing it electrode, which generates a sensing signal while being charged/discharged according to the control clock; a second charge/discharge unit connected in parallel to the first charge/discharge unit, which generates a reference signal while being charged/discharged according to the control clock; and a detection unit for detecting a change in the capacitance on the side of the sensing unit electrode by comparing the sensing signal from the first charge/discharge unit with the reference signal from the second charge/discharge unit. The first charge/discharge unit includes: a first capacitor connected at one end thereof to the sensing unit electrode, which is charged/discharged according to the control clock; a first constant-current source for supplying a predetermined amount of constant-current to the first capacitor, which charges the first capacitor; and a first switch for controlling the first capacitor such that, according to the control clock, the first capacitor is repetitively charged and discharged every half cycle of the clock. The second charge/discharge unit includes: a second capacitor which is charged/discharged according to the control clock; a second constant-current source for supplying a predetermined amount of constant-current to the second capacitor so as to charge the second capacitor; and a second switch for controlling the second capacitor such that, according to the control clock, the second capacitor is repetitively charged and discharged every half cycle of the clock.

FIELD

The present invention relates to a capacitive sensor for detecting acapacitance, which is generated due to the proximity of an object, by aproximity sensor or the like, and more particularly, a high-sensitivitycapacitive sensor circuit, in which a plurality of detection circuitsusing charging and discharging of a capacitor are provided, one of thedetection circuits is used for a reference voltage, and a peak of asensing signal is detected, thereby improving detection sensitivity.

BACKGROUND

In general, a proximity sensor is a sensor capable of detecting theproximity of an object to be detected in a non-contact manner, and isclassified into a high-frequency oscillation type using electromagneticinduction according to a detection method, a capacitive type for sensinga change in capacitance between an object to be detected and a sensor,and a magnetic type using a magnet. In addition, a proximity sensor maybe classified into a shield type (embedded type) and an unshielded type(protrusion type) according to a structure of a sensing head orclassified into a DC 2-wire type, a DC 3-wire type (NPN, PNP), an ACcontactless type, etc. according to an output circuit.

A high-frequency oscillation type proximity sensor generates ahigh-frequency magnetic field through a sensing coil at a front end ofthe proximity sensor, and detects a reduction or stopping of oscillationdue to an overcurrent caused by an induced magnetic field generated in ametal due to electromagnetic induction when an object to be detectedapproaches a magnetic field. A capacitive proximity sensor includes aninduction electrode of a sensing unit, and detects a large change incapacitance between the induction electrode of the sensing unit and theground when an object approaches the induction electrode. The capacitivetype proximity sensor has a fast response rate and is capable ofdetecting objects other than metals and thus has been widely used inindustrial fields.

‘Capacitance Detection Proximity Sensor’ which is a capacitive proximitysensor of the related art is disclosed in registered Patent No.10-1046666 published in Registered Patent Publication B1. The proximitysensor includes a first sensing electrode and a second sensing electrodewhich are arranged spaced a certain distance from each other in adetection direction in which an object to be detected approaches andoperate independently from a ground potential, and a proximity detectioncircuit that outputs, as a proximity detection output, the differencebetween a ground capacitance formed by the first sensing electrode and aground capacitance formed by the second sensing electrode; and isconfigured to limit a proximity detection range to an open area andreduce erroneous operations.

In ‘Capacitive Proximity Sensor and Proximity Detection Method’disclosed in Korean Patent Application No. 10-2010-0100773 published inLaid-Open Patent Publication A, a sensing unit includes a sensorelectrode, a shield electrode, and an auxiliary electrode, the sensorelectrode is connected to a capacitance-to-voltage (C-V) conversioncircuit, the shield electrode is connected to a shield driving circuit,and the auxiliary electrode is connected to the C-V conversion circuitor the shield driving circuit through a changeover switch.

In the capacitive proximity sensor of the related art, a detectioncircuit configured to detect a change in capacitance is more likely tobe influenced by noise when an amplification degree is increased andthus it is difficult to implement a high-sensitivity capacitive sensor.For example, as illustrated in FIG. 5, a capacitive sensor circuit ofthe related art includes an oscillator 31, a mixer 32, a low-pass filter(LPF) 33, and an oscillation sensor 34 for detecting a capacitancesensed by a sensing unit connected thereto via a sensing unit electrode35 and a capacitance of a capacitor C1, and detects a change incapacitance on the basis of a change in oscillation frequency of theoscillation sensor 34 caused by a change in capacitance of the sensingunit. Referring to FIG. 5, in the capacitive sensor circuit of therelated art, when an oscillation signal from the oscillation sensor 34and an oscillation signal from the oscillator 31 are input to the mixer32, a difference signal among the sum of and a difference signal betweentwo signals output from the mixer 32 is low-pass filtered by the LPF 3to detect a change in capacitance.

However, when capacitance-to-voltage conversion is performed based on achange in frequency as described above, a reference voltage changes dueto circuit imbalance and characteristics change over temperature,thereby making it difficult to implement a high-sensitivity capacitivesensor. That is, in the related art, when an amplification degree isincreased to increase sensitivity, noise increases and thus an erroneousoperation is likely to occur.

SUMMARY Technical Problem

To address the above-described problem, the present invention isdirected to providing a high-sensitivity capacitive sensor circuit inwhich a plurality of detection circuits using charging/discharging of acapacitor are provided, one of the detection circuits is used for areference voltage and a peak of a sensing signal is detected, therebyimproving detection sensitivity.

Technical Solution

According to one aspect of the present invention, a high-sensitivitycapacitive sensor circuit includes an oscillator configured to generatea control clock; a first charge and discharge unit connected to asensing unit electrode (a terminal of a sensor for sensing a capacitanceof an object) and configured to generate a sensing signal while beingcharged or discharged according to the control clock; a second chargeand discharge unit connected in parallel to the first charge anddischarge unit and configured to generate a reference signal while beingcharged or discharged according to the control clock; and a sensorconfigured to detect a change in capacitance at the sensing unitelectrode by comparing the sensing signal from the first charge anddischarge unit with the reference signal from the second charge anddischarge unit.

The first charge and discharge unit may include a first capacitorconfigured to be charged or discharged according to the control clock,wherein the sensing unit electrode is connected to one end of the firstcapacitor; a first constant-current source configured to supply aconstant current having a certain magnitude to the first capacitor so asto charge the first capacitor; and a first switch configured to controlthe first capacitor to be repeatedly charged and discharged every halfclock cycle according to the control clock.

The second charge and discharge unit may include a second capacitorconfigured to be charged or discharged according to the control clock; asecond constant-current source configured to supply a constant currenthaving a certain magnitude to the second capacitor so as to charge thesecond capacitor; and a second switch configured to control the secondcapacitor to be repeatedly charged and discharged every half clock cycleaccording to the control clock.

The first constant-current source and the second constant-current sourcemay supply constant currents having the same magnitude, and the constantcurrents may be generated through a constant current drive.

The sensor may include a first peak sensor configured to detect a peakvalue of the sensing signal from the first charge and discharge unit; asecond peak sensor configured to detect a peak value of the referencesignal from the second charge and discharge unit; a subtractorconfigured to calculate the difference between the peak values throughsubtraction between an output of the first peak sensor and an output ofthe second peak sensor; and an amplifier configured to amplify an outputof the subtractor.

Advantageous Effects

In a capacitive sensor circuit according to the present invention, twodetection circuits using charging/discharging of a capacitor areprovided, one of the detection circuits is used for a reference voltageand a peak of a sensing signal is detected to overcome the influence ofnoise, thereby improving detection sensitivity to be at least severaltimes that of the related art.

In addition, according to the present invention, the linearity of asensing signal can be improved, the influence of temperature can beminimized, and a detection distance can be increased up to 300 mmdepending on the application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the concept of capacitancedetection using charging and discharging according to an embodiment ofthe present invention,

FIG. 2 is a waveform diagram of a clock and a sensing signal illustratedin FIG. 1,

FIG. 3 is a block diagram of a capacitive sensor circuit according to anembodiment of the present invention,

FIG. 4 is an operational waveform diagram of a capacitive sensor circuitaccording to an embodiment of the present invention, and

FIG. 5 is a schematic diagram of a general capacitive sensor circuit.

DETAILED DESCRIPTION

Aspects of the present invention achieved by implementing embodiments ofthe present invention will be more apparent from the embodimentsdescribed below. These embodiments are only examples provided to explainthe present invention and are not intended to limit the scope of thepresent invention.

FIG. 1 is a schematic diagram illustrating the concept of capacitancedetection using charging and discharging according to an embodiment ofthe present invention. FIG. 2 is a waveform diagram of a clock and asensing signal illustrated in FIG. 1.

As illustrated in FIG. 1, in the concept of capacitance detection usingcharging and discharging according to an embodiment of the presentinvention, a switch SW turned on/off by a clock is connected in parallelto a charge/discharge capacitor C and a constant-current source I isconnected in series to the capacitor C, and thus, the capacitor C ischarged with current I flowing from the constant-current source I whenthe switch SW is off and is discharged when the switch SW is on. In thiscase, a sensing unit is connected to one end of the charge/dischargecapacitor C and detects a change in capacitance by detecting a charge incharging waveform of the capacitor C due to a sensed capacitance C′formed between an object to be detected and the sensing unit.

As illustrated in FIG. 2, a clock for control of charging anddischarging of the capacitor C is a square wave having a certain cycleand a voltage charged in the capacitor C is a saw tooth wave having acertain cycle and a peak voltage V. Referring to FIG. 2, a sawtooth waveis generated as the capacitor C is charged every square-wave half cyclet.

Referring to FIGS. 1 and 2, a charging voltage V is calculated bydividing the quantity of electric charge It for a time t by acapacitance of the capacitor C as shown in Equation 1, and the time t ishalf a cycle of the clock.

$\begin{matrix}{V = {\frac{1}{C}{It}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

For example, when a peak value is 2.5 V and a clock frequency is 100Khz, a constant current I is calculated to be 5 μA according to Equation1 above. When a detected capacitance C′ generated by a hand or the likeis 1 pF, a peak voltage V′ is calculated to be approximately 2.27 Vaccording to Equation 2 below.

$\begin{matrix}{V^{\prime} = {\frac{1}{C + C^{\prime}}{It}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Therefore, in an example of the circuit described above, a change rateis calculated to be (1−2.27/2.5), i.e., approximately 9.2%. To increaseactual detection sensitivity, it is necessary to increase a peak voltageand reduce a capacity of a capacitor.

FIG. 3 is a block diagram of a capacitive sensor circuit according to anembodiment of the present invention. FIG. 4 is an operational waveformdiagram of a capacitive sensor circuit according to an embodiment of thepresent invention.

A capacitive sensor circuit according to an embodiment of the presentinvention includes an oscillator for generating a control clock, a firstcharge and discharge unit connected to a sensing unit electrode andconfigured to generate a sensing signal while being charged ordischarged according to the control clock, a second charge and dischargeunit connected in parallel to the first charge and discharge unit andconfigured to generate a reference signal while being charged ordischarged according to the control clock, and a sensor configured todetect a change in capacitance at the sensing unit electrode bycomparing the sensing signal from the first charge and discharge unitwith the reference signal from the second charge and discharge unit.

As illustrated in FIG. 3, a capacitive sensor circuit according to anembodiment includes a first capacitor C1, a second capacitor C2, anoscillator 101, a first switch 102 d, a second switch 102 r, aconstant-current drive 103, a first constant-current source 104 d, asecond constant-current source 104 r, a first buffer 105 d, a secondbuffer 105 r, a first peak sensor 106 d, a second peak sensor 106 r, anoffset adjuster 107, a subtractor 108, an amplifier 109, an outputterminal 110, an offset coordinator 111, a comparator 112, and a sensingunit electrode 113. The oscillator may be embodied as the oscillator101. The first charge and discharge unit may be embodied as the firstcapacitor C1, the first switch 102 d, and the first constant-currentsource 104 d. The second charge and discharge unit may be embodied asthe second capacitor C2, the second switch 102 r, and the secondconstant-current source 104 r. The sensor may be embodied as the firstpeak sensor 106 d, the second peak sensor 106 r, the subtractor 108, andthe amplifier 109.

Referring to FIG. 3, the oscillator 101 generates a square-wave pulseclock having a cycle T (a half cycle t), as illustrated in FIG. 4 (b),and provides the square-wave pulse clock to both the first switch 102 dand the second switch 102 r.

One end of the first capacitor C1 is connected to the sensing unitelectrode 113, the first constant-current source 104 d, and the firstswitch 102 d, and the other end is connected to a ground voltage GNDsource. As illustrated in FIG. 4, the first switch 102 d is turned offfor the half cycle t of the clock to charge the first capacitor C1 withcurrent i1 from the first constant-current source 104 d, therebygenerating a sawtooth wave, and is turned on for another half cycle ofthe clock to discharge the first capacitor C1.

One end of the second capacitor C2 is connected to the secondconstant-current source 104 r and the second switch 102 r and the otherend is connected to the ground voltage GND source. As illustrated inFIG. 4, the second switch 102 r is turned off for the half cycle t ofthe clock to charge the second capacitor C2 with current i2 from thesecond constant-current source 104 r to generate a sawtooth wave, and isturned on for another half period of the clock to discharge the secondcapacitor C2. In this case, the constant currents i1 and i2 having thesame magnitude (i.e., i1=i2) flow from the first constant-current source104 d and the second constant-current source 104 r to the capacitors C1and C2 through the same constant-current drive 103.

Referring to FIG. 4, (a) illustrates a charging voltage waveformgenerated by the first capacitor C1 or the second capacitor C2, and (b)illustrates a clock waveform supplied to the first switch 102 d and thesecond switch 102 r.

When no object is sensed by a sensing unit (not shown), a sensedcapacitance C′ at the sensing unit electrode 113 is 0, and the constantcurrents i1 and i2 having the same capacitance value and the samemagnitude (here, i1=i2) respectively flow to the first capacitor C1 andthe second capacitor C2, and therefore, charging voltages V are chargedwith the same constant current for a half cycle of a clock and thus havethe same value. That is, a charging voltage of the second capacitor C2acts as a reference voltage and a charging voltage of the firstcapacitor C1 acts as a sensing voltage. When no object is sensed by thesensing unit, C′=0 and thus C2=C1 and i1=i2. Therefore, the chargingvoltage of the first capacitor C1 and the charging voltage of the secondcapacitor C2 are calculated to be the same according to Equation 1 aboveand thus the sensing voltage and the reference voltage are the same.When the sensing voltage and the reference voltage are different fromeach other due to a characteristic deviation of a circuit elementalthough no object is sensed by the sensing unit, the offset adjuster107 is controlled by the offset coordinator 111 to perform calibrationsuch that detection values are the same (i.e., the difference betweenthe detection values is zero).

On the other hand, when an object is sensed by the sensing unit, thesensed capacitance C′ of the sensing unit is connected through thesensing unit electrode 113 and is connected in parallel to the firstcapacitor C1, thereby increasing the total capacitance of the firstcharge and discharge unit. Therefore, a charging voltage V′ when theobject is sensed may be calculated using Equation 2 above, and is lowerthan a charging voltage of the second capacitor C2 and thus the objectmay be sensed according to the difference (V−V′=ΔV) between the chargingvoltage V′ and the charging voltage V.

Referring back to FIG. 3, the first buffer 105 d buffers a chargingvoltage (i.e., a sensing voltage) of the first charge and discharge unitand transmits the buffered voltage to the first peak sensor 106 d, andthe second buffer 105 r buffers a charging voltage (i.e., a referencevoltage) of the second charge and discharge unit and transmits thebuffered voltage to the second peak sensor 106 r.

The first peak sensor 106 d detects a peak value of the sensing voltage,the second peak sensor 106 r detects a peak value of the referencevoltage, and the subtractor 108 calculates the difference between thepeak value of the reference voltage and the peak value of the sensingvoltage and outputs the difference to the amplifier 109. The amplifier109 amplifies the output of the subtractor 108 and outputs the amplifiedoutput to the outside via the output terminal 110.

As described above, the offset adjuster 107 is controlled by the offsetcoordinator 111 to calibrate an offset such that an output signal of thefirst charge and discharge unit and an output signal of the secondcharge and discharge unit are the same in a state in which the object isnot detected.

The comparator 112 compares a peak value of the reference voltage with aset voltage V so that the constant currents i1 and i2 may flow throughthe first capacitor C1 and the second capacitor C2 for only a half cycle(a charge period) of a clock, and controls the constant-current drive103 to prevent the constant currents i1 and i2 from flowing through thefirst capacitor C1 and the second capacitor C2 for a discharge period.

In an embodiment of the present invention, the same current is suppliedto constant-current circuits of two sensors (charge and discharge units)so that directions of current noise may be the same. Therefore, theinfluence of the current noise may be reduced through comparison(subtraction) between the reference voltage and the sensing voltage, anddrift and the like may be eliminated similarly.

While the present invention has been described above with reference toone embodiment illustrated in the drawings, it will be understood bythose of ordinary skill in the art that various modifications may bemade therein and equivalent other embodiments are possible.

1. A high-sensitivity capacitive sensor circuit comprising: anoscillator configured to generate a control clock; a first charge anddischarge unit connected to a sensing unit electrode and configured togenerate a sensing signal while being charged or discharged according tothe control clock; a second charge and discharge unit connected inparallel to the first charge and discharge unit and configured togenerate a reference signal while being charged or discharged accordingto the control clock; and a sensor configured to detect a change incapacitance at the sensing unit electrode by comparing the sensingsignal from the first charge and discharge unit with the referencesignal from the second charge and discharge unit.
 2. Thehigh-sensitivity capacitive sensor circuit of claim 1, wherein the firstcharge and discharge unit comprises: a first capacitor, one end of whichis connected to the sensing unit electrode and which is charged ordischarged according to the control clock; a first constant-currentsource configured to supply a constant current having a certainmagnitude to the first capacitor so as to charge the first capacitor;and a first switch configured to control the first capacitor to berepeatedly charged and discharged every half clock cycle according tothe control clock.
 3. The high-sensitivity capacitive sensor circuit ofclaim 1, wherein the second charge and discharge unit comprises: asecond capacitor configured to be charged or discharged according to thecontrol clock, a second constant-current source configured to supply aconstant current having a certain magnitude to the second capacitor soas to charge the second capacitor; and a second switch configured tocontrol the second capacitor to be repeatedly charged and dischargedevery half clock cycle according to the control clock.
 4. Thehigh-sensitivity capacitive sensor circuit of claim 2, wherein the firstconstant-current source and the second constant-current source supplyconstant currents having the same magnitude, wherein the constantcurrents are generated by a constant current drive.
 5. Thehigh-sensitivity capacitive sensor circuit of claim 1, wherein thesensor comprises: a first peak sensor configured to detect a peak valueof the sensing signal from the first charge and discharge unit; a secondpeak sensor configured to detect a peak value of the reference signalfrom the second charge and discharge unit; a subtractor configured tocalculate the difference between the peak values through subtractionbetween an output of the first peak sensor and an output of the secondpeak sensor; and an amplifier configured to amplify an output of thesubtractor.
 6. The high-sensitivity capacitive sensor circuit of claim3, wherein the first constant-current source and the secondconstant-current source supply constant currents having the samemagnitude, wherein the constant currents are generated by a constantcurrent drive.